For example, as an inverter for an electric motor vehicle, a semiconductor module, for example, an IGBT (Insulated Gate Bipolar Transistor) module which enables high voltage and large current operations is used. In such a semiconductor module, a semiconductor chip is bonded to a circuit board having a high electrical insulation property and high mechanical strength. In the circuit board, a metal circuit plate serving as a wiring is formed on an insulating ceramic substrate. The semiconductor chip is mounted on the circuit board and wiring on the semiconductor chip is electrically connected to the metal circuit plate. The electric connection is established by the method using a bonding wire made of aluminum or gold, or by the method using a solder bump or a solder ball.
High reliability is required for the electrical connection. That is, it is necessary that, even in a high temperature environment, mechanical strength for the connection can be kept high and an electrical resistance at a bonded portion can be kept low. An electrode structure in which the above points have been improved is disclosed in Patent Reference 1.
FIG. 8 is a cross-sectional diagram showing a semiconductor module 80 employing the electrode structure and an enlarged diagram showing a cross section of the electrode structure. Here, in the circuit board 90, a metal circuit plate 92 made of cupper (Cu) is formed on a ceramic substrate 91 on which a semiconductor chip 81 is mounted. The semiconductor chip 81 and metal circuit plate 92 are connected with each other by a bonding wire made of aluminum (Al). In a bonded portion between the metal circuit plate and bonding wire 93 is mounted a coating layer 94 for improving the connection therebetween. The coating layer 94, as shown by the enlarged diagram in FIG. 8, is made up of a nickel (Ni) layer 941, a palladium (Pd) layer 942, and a gold (Au) layer 943, which are stacked upward from bottom. A phosphorous (P) layer 941 is added to the Pd layer 942. Each of the Ni layer 941, Pd layer 942, and Au layer 943 is formed by plating.
The bonding wire 93 is, for example, about 100 μm in diameter, and the connection using the bonding wire 93 is performed by ultrasonic bonding. At this time, ultrasonics are applied to the bonding wire 93 in a state where the metal circuit plate 92 (coating layer 94) is adhered with pressure to be bonded.
In the structure described above, an alloy layer is formed at an interface between Au and Al, thus increasing bonding strength between the bonding wire 93 and Au layer 943. Owing to low electrical resistance of the Au layer 943, excellent electrical characteristics can be obtained. Since the alloy is formed also at the interface between Cu and Ni making up the metal circuit plate 92, the bonding strength between the metal circuit plate 92 and Ni layer is high. Au/Pd and Ni/Pd form an alloy and, therefore, bonding strength between the Au layer 943 and Pd layer 942 and between the Pd layer 942 and Ni layer 941 is also large. Therefore, in this structure, adhesion property at each interface is excellent.
In the above structure, Cu or Ni diffused into the Au layer 943 degrades the bonding strength between the bonding wire 93 and Au layer 943. Particularly, this kind of the diffusion occurs in a thermal process such as soldering or molding in which a temperature of 300° C. or higher is applied. The Pd layer 942 plays a role of suppressing the diffusion. Moreover, by doping P to the Pd layer 942, the oxidation of the Ni layer 941 can be suppressed.
Therefore, by performing the wire bonding on the electrode structure, high and stable bonding strength in the wire bonding is obtained and highly reliable semiconductor module can be provided.
Moreover, this electrode structure is used in a voltage controlled signal transmitter, antenna switch, high-frequency amplifier or the like employed in wireless communication devices such as a mobile phone, GPS (Global Positioning System), LAN (Local Area Network) and high frequency modular electronic components obtained by integrating these components. In the high-frequency electronic components, semiconductor elements serving as active elements and electronic devices such as a chip inductor, chip capacitor, and chip resistor are surface-mounted on a ceramic layer stacked substrate internally having an electronic circuit formed by a conductor pattern. The high frequency circuit portion (RF circuit) of the high frequency electronic components handles a high frequency signal of several hundred MHz to several GHz.
FIG. 9 is a cross-sectional view showing a conventional high frequency electronic component. The high frequency electronic component 30 is a high frequency wave amplifier and has at least a multi layer substrate 40 and a semiconductor device 31 and, if necessary, an electronic component 45. The multi layer substrate 40 is mainly made up of a plurality of ceramic layers 41 in which multi-layers are integrated by being sintered and of a conductor pattern 44. The conductor pattern 44 includes an electrode on which a semiconductor device 31 formed on a bottom of a cavity 46 provided on a main surface on a surface side of the multi layer substrate 40 is mounted, a terminal electrode and ground pattern formed on the main surface on a rear side of the cavity, a plurality of thermal vias to connect the ground pattern with the electrode, an internal conductor pattern making up a capacitor element and inductor element, line paths connecting these components, via holes, and the like. The electrode (pad) 42 being connected to the conductor pattern 44 is formed on a main surface on a surface side of the multi layer substrate 40. The semiconductor element 31 is housed in the cavity 46 formed on the multi layer substrate 40 and is electrically connected by the electrode 42 and bonding wire 43 formed in a portion surrounding the cavity 46.
The electrode 42, as shown in the enlarged diagram in FIG. 9, is made up of a base electrode 420 made of, for example, Ag, and of a coating layer 421 to coat the surface of the base electrode 420. The coating layer 421 includes the Ni layer 421a to coat the base electrode 420, the Pd layer 421b to coat the Ni layer, and the Au layer 421c to coat the Pd layer. By performing wire bonding on the electrode structure, it was possible to obtain the high frequency electrode component having high reliability and highly stable bonding strength.    Patent Reference 1: Japanese Patent Application Laid-open No. 2005-72282.